Generally, for writing instruments such as oil-based ball-point pens, water-based ball-point pens, a removable cap for protecting the front end of the writing part by enclosing it when unused is provided.
Particularly, in some kinds of writing instruments, in order to avoid the writing part drying, the tip part is hermetically enclosed by the cap so that the part can be isolated from the outside air.
However, there is a hazard risk such as from choking if an infant accidentally swallows the cap which is of a hermetic type with no ventilation.
To avoid this, various cap configurations have been known: for example, a cap is composed of outer and inner sleeves with a vent hole for ventilation provided in their connecting portion; and the external form of a cap body is shaped with projections and indentations so as to form gaps for ventilation.
Specifically, there is a configuration in which a vent hole is formed at the front end of the cap, as disclosed in Japanese Patent Application Laid-open Hei 8 No. 197889 and Japanese Patent Application Laid-open Hei 9 No. 169196. There is another proposal in which a vent hole is formed on the cap's outer periphery so as to establish communication for ventilation between the interior and exterior of the cap, as disclosed in Japanese Patent Application Laid-open No.2000-43477.
Further, Japanese Registered Utility Model No.2579697 has disclosed a configuration in which a cap having an integral clip is formed so that the sectional shape of the clip attachment is constricted in the middle, whereby the constricted part would create air channels in case an infant accidentally swallowed the cap.
However, the above configurations suffer difficulties in the forming process because of their cap shape complexity and difficulties in assembly because of their structure complexity. In addition, formation of a vent hole in the outer periphery spoils the outside appearance.
The present invention has been devised in view of the above conventional problems, it is therefore an object of the present invention to provide a writing instrument cap which is simple in structure and can assure air passage in case of an accidental swallowing, without any loss of external appearance.